New research shows that global warming could cause the world's polar ice sheets to melt faster than expected. Scientists predict the melting ice will eventually cause sea levels to rise up to six meters, threatening human activity along coasts and on islands.

The general consensus among climate scientists is that the average global temperatures will rise 1.5 to six degrees Celsius by the end of this century. This is roughly as warm as the Earth got about 129,000 years ago, the time between the most recent ice age and the previous one.

Researchers using modern computer models and natural climate records, such as data from ancient coral reefs and samples of deep ice, have found that sea levels at that long-ago time were about four to six meters higher than now. They report in the journal Science that the cause of the ancient sea level rise was significant melting of the Greenland and Antarctic ice sheets.

University of Arizona geoscientist Scott Overpeck says that when the computer models are fed the future temperature estimates, they show that the ice sheets would repeat their past performance. Ocean levels would rise one meter by the year 2100 and continue rising for centuries afterward.

"Right now our sea level rise is proceeding at about three millimeters per year," he noted. "That is mostly due to ocean expansion as the ocean warms. This will continue and it will result ultimately in at least a meter of sea level rise if we don't do anything. What we're talking about is melting of ice sheets, which will be on top of this and which will accelerate the rate of sea level rise."

The study follows recent reports of satellite observations showing that the Greenland and Antarctic ice sheets have been losing area to the sea for several years, by melting and large chunks breaking off.

A second study in the journal Science shows that at least part of this loss apparently occurs from beneath the ice sheets because of warmer ocean water at intermediate depths. U.S. space agency researcher Robert Bindshadler says the warm water might be melting the edges of the glaciers from below and causing them to move faster.

"There's a big temperature contrast between the warm ocean water and the cold ice, and melting occurs at a very rapid rate," he noted. "It reduces the friction that holds these glaciers back, allowing them to accelerate."

Scientists attribute the temperature increase behind the melting glaciers to an increase in carbon dioxide and other so-called greenhouse gases in the atmosphere, emitted largely by industry and automobiles. Scott Overpeck at the University of Arizona says this differs from the temperature rise 129,000 years ago, which was due to a greater tilt of the northern hemisphere toward the sun.

"What we're doing -- humans -- to the climate system will be much harsher on ice sheets than what Mother Nature did 129,000 years ago," he explained. "Back then it was warming primarily in the northern hemisphere in the summer. Now we're warming the entire Earth and we're doing it year 'round."

As a result, says Overpeck, the polar ice sheets probably will be more susceptible to warming in the future than they were in the past.